Cardiac arrhythmias are the leading cause of death in the world. In most cases, the ultimate events underlying arrhythmias are disturbances in function of various ion channels and pumps. Recent studies have undoubtedly shown that IK1, the major inwardly rectifying potassium current in the heart, is a critical player in cardiac excitability. Mutations in human Kir2.1, a gene underlying IK1, were found to be the cause of Long QT (Andersen's) and Short QT (SQT3) syndromes, and familial atrial fibrillation. Additionally, experiments with transgenic animal models show that manipulations of IK1 lead to altered excitability, with IK1 upregulation being proarrhythmic. Nevertheless, the mechanisms behind IK1 involvement in arrhythmogenesis remain largely unknown. Since IK1 plays a significant role in shaping the late phase of action potential (AP) repolarization and stabilization of resting potential, its heterogeneous expression may be an important factor contributing to dispersion of cardiac repolarization, a parameter considered to be essential in determining the electrical stability of the heart. Unfortunately, one of the impediments in studying cardiac excitability is that specific ionic conductances cannot be directly measured in intact hearts using electrophysiological tools. Classical radioactive ion flux assays present a principally different alternative, but their potential has been exhausted. This project is specifically designed to make significant advancement in determining the spatial distribution of IK1 in intact isolated hearts by combining the unique properties of IK1 and non-radioactive thallium flux assays. It will develop new and adapt established technologies for studying IK1 allowing for the creation of a detailed spatial map of IK1 conductance. The successful completion of this study will greatly advance our understanding of the new roles of IK1 in cardiac excitability and create an opportunity to monitor IK1 during various physiological interventions. Therefore, this study will contribute significantly to the development of novel approaches in the treatment and prevention of cardiac arrhythmias. PUBLIC HEALTH RELEVANCE: This project will develop new and adapt established ion flux technologies for studying the major cardiac inward rectifier potassium current, IK1, a critical component of cardiac excitability. Results of this project will greatly advance our understanding of its role in cardiac excitability and therefore contribute significantly to the development of novel approaches in the treatment and prevention of devastating cardiac arrhythmias.